1. Field of the Invention
The present invention relates generally to an accelerometer and more specifically to a semi-conductor type accelerometer which is suited for use in vehicular control systems and the like.
2. Description of the Prior Art
FIGS. 1 to 3 show a prior art arrangement disclosed in IEEE ED Vol. Ed-26 No. 12 page 1911 to 1917 (1979). This arrangement includes a G responsive weight or mass 1 which is supported on the main body 2 of a silicon type semi-conductor substrate by a relatively thin flexible beam portion or section 4. Piezo resistors Ra and Rb are formed on the upper surfaces of the beam 4 and main body 2 of the substrate respectively, by doping. Heavily doped regions 6, 8 interconnect the resistors and act as terminals via which they can be connected to other circuity.
Stepped stopper members 10 (only the lower one is illustrated in FIG. 3) are secured to the upper and lower faces of the main body of the substrate via anodic bonding. These members 10 shield the resistors Ra, Rb from external environmental moisture, strain and the like and further function to support the actual sensor element in a manner which permits the G responsive element 1 to move within acceptable limits in response to the application of accelerative forces.
When an acceleration is applied to the sensor the beam portion flexes so that the surface on which resistor Ra is formed distorts through an angle .alpha. (see FIG. 5). Viz., the force (f) applied to the G responsive element can be represented by: EQU f=m.times..alpha. (1)
wherein m denotes the effective mass of the G responsive element.
White this prior art device it is necessary to construct the associated bridge circuit in a manner as shown in FIG. 4.
The output Vout of this circuit is given by: EQU Vout=(1/4).multidot.(.alpha.R/R).multidot.Vdd (2)
wherein:
.DELTA.R denotes the fractional resistance change due to the acceleration; PA1 R=Ra=Rb=Rc (under zero acceleration).
Vdd denotes the voltage applied across the circuit; and
However, this arrangement has suffered from the following drawbacks:
1. Among the four resistors (Ra, Rb, Rc, Rc) only the resistance of Ra changes with acceleration due to the stress induced in the beam portion, and as will be appreciated from equation (2), the variation in the output Vout is only 1/4 the change in the resistance of Ra. Hence, the sensitivity of this device is low.
2. As no protecting layer or film is formed on the piezo resistors the reliability and stability thereof is poor;
3. Due to the construction wherein the stopper members are bonded to either side of the substrate on which the measuring electrodes are formed, external temperature has a greater effect on the resistor Rb formed on the main body than on the resistor Ra which is partially insulated due to its disposition on the relatively thin beam section. Thus, with changes in external temperature the resistance values of resistors Ra and Rb tend to become offset and induce the output of the circuit to drift in a manner which reduces the accuracy of the device.
4. During the production of the device an oxide layer 14 is formed on the surface of the silicon substrate (see FIG. 5). However, the expansion coefficients of the substrate and the oxide layer are different and, as shown in FIG. 5, the thermal stress which develops in the beam section of the accelerometer is larger than that in the main body of the substrate. This difference causes a temperature differential between the resistors Ra and Rb and has a detrimental influence on the accuracy of the sensor.